Electron injection plasma variable reactance device



April 15, 1969 YASUO WADA 3,439,223

ELECTRON INJECTION PLASMA VARIABLE REACTANCE DEVICE Filed Oct. 24, 1966 Sheet of 45 6471/02: AMA/A 41000524444 T O/nuke: Z 4 2'3 ADJUSTABLE 1 POTENTIAL v I sauna: 7/ l 5 .Zizaa5 \74 1 n'llk" a9 ADJUSTABLE POTENTIAL SOURCE Away/v4 KHz/0 W404, a/

April 15, I969 YASUO WADA Sheet ADJUS 748. E P0 T E N T ML SOURCE ziza 2.

SOURCE ADJUSTABL E P0 T E N TIA L 7 E am: A 6 TNR SE www 0 P April 15, 1969 YASUO wAbA ELECTRON INJECTION PLASMA VARIABLE REACTANCE DEVICE Sheet Filed Oct. 24, 1966 ADJUSTABLE POTENTIAL SOURCE z za a.

T E /0 am mu R WWW up A United States Patent U.S. Cl. 315-39 12 Claims ABSTRACT OF THE DISCLOSURE Plasma is produced in a waveguide for presenting variable reactance to radio frequency energy transmitted along the waveguide. The plasma is electron injected and is variable to present variable reactance to the RF energy.

Background This invention relates to a variable reactance device and more particularly to ,a plasma variable reactance device providing electronically controllable variable reactance to radio frequency electromagnetic energy.

The use of variable reactance devices as either fixed or controllable reactance devices is well established in the art. For example, variable reactance devices have been used for electronic control of antenna arrays, especially slot antenna arrays; electronic switching of moderate to high power microwave energy; frequency multiplication; parametric amplification; electronically controlled phase shifters, to name just a few.

Variable reactance devices are generally classified into three categoriessolid-state or semiconductor varactors, ferrite variable reactance devices and plasma variable reactance devices.

For low radio frequency (RF) power levels, solidstate variable reactance devices provide useful electronically controlled RF electromagnetic energy reactances, especially at microwave frequencies. Typical characteristics of a good solid-state variable reactance device are a quality factor or Q of the order of at X-band but a microwave power handling capability of the order of only one watt.

Ferrite devices have been commonly used for electronic switching and/or phase control of microwaves but are somewhat limited in the amount of average RF power they can handle. Also, these devices introduce significant RF losses and are temperature sensitive to a substantial degree.

Variable reactance devices utilizing the plasma of a gas discharge are called plasma variable reactance devices and also are capable of providing an electronically controllable variable reactance. This type of variable reactance device is generally less efficient than solid-state variable reactance devices, introduced a considerable amount of noise to the system and have a relatively low quality factor or Q.

The electron injection plasma variable reactance devices of the invention on the other hand have the advantage of being able to effectively operate at relatively high average RF power levels and introduce lower RF losses than ferrite devices, for example, and also are less temperature sensitive. Variable reactance devices constructed according to the invention furthermore have a relatively lower equivalent noise temperature, a much higher Q and lower insertion loss than conventional plasma variable reactance devices.

Accordingly, it is an object of this invention to provide an improved electronically controllable radio frequency electromagnetic energy reactance device.

It is another object of the invention to provide a more ice efiicient plasma variable reactance device than heretofore obtainable.

It is still another object of the invention to provide a plasma variable reactance device wherein the plasma is maintained substantially free from unwanted oscillations and has a low equivalent radio frequency noise temperature.

It is yet another object of this invention to provide it variable reactance device with a very low RF insertion oss.

It is a further object of the invention to provide a variable reactance device having a relatively high Q.

Itis' still a further object of the invention to provide a variable reactance device having a high RF power ham dling capability.

Summary The above mentioned and other objects of the invention are achieved in an electron injection plasma variable reactance device adapted to interact with radio frequency electromagnetic energy. According to one embodiment of the invention, the variable reactance device comprises an electron emitter (cathode), an electron collector (anode) spaced from the emitter and a gaseous medium maintained in theregion between the emitter and the collector. The emitter and collector are connected to an appropriate adjustable potential source to create a plasma (cathode) sheath having an injection boundary from which electrons are injected into the gas. The injected electrons travel a mean distance L from the injection boundary to the collector surface whereby the gas is ionized to form a plasma of a density dependent upon the magnitude of the potential difference between the plasma sheath and the collector and upon the discharge current. The gas is maintained at a pressure such that the ionization mean free path for the injected electrons is at least of the order of the distance L. Also, the device is arranged so that there is an interaction between the radio frequency electromagnetic energy to be influenced and the plasma.

The invention and specific embodiments thereof will be described hereinafter by way of example and with reference to the accompanying drawings wherein like reference numerals refer to like elements and parts,

Description of the drawing FIG. 1 is a potential distribution curve showing how, according to the invention, electron injection takes place through a cathode sheath;

Description In order to facilitate in the description of the invention, the following material relating to plasma variable reactance devices in general should be noted.

A plasma variable reactance device is a device producing a plasma of controlled density over an appropriate volume. The plasma acts as a dielectric Whose dielectric constant e is determined by the electron density n of the plasma and is given by Heald and Wharton at page 6 where n =electron density e=electron charge m -=electron mass 7=e1ectron collision frequency w=21rf (RF field frequency) e =dielectric constant of free space It is seen that an increase in electron density n results in a reduction of dielectric constant e, which means essentially that the plasma behaves as an inductive medium, the inductive effect of the plasma being controlled by controlling n As an example, at X-band (f= c.p.s.), in order to make 6 0, it can be shown that a plasma density n =10 electrons/cm. will be required. This value can readily be obtained by means of gas discharges.

The RF losses of a plasma variable reactance device can be calculated by means of Equation 1 and are best expressed in terms of the Q of the reactance presented by the plasma to the RF fields. Q is defined as:

=wW/P (2) whe re W=kinetic RF energy stored in plasma P =RF powe dissipated in plasma where E=RMS RF electric field. From the above relationship and Equation 1:

Equation 3 shows that in order to make a high Q (low RF loss) plasma variable reactance device, it is necessary to use a gas discharge in which the collision frequency 7 is low. In the type of discharge devices to be considered, electron-neutral collisions predominate over electron-ion collisions and therefore the problem is to minimize the electron-neutral collision frequency. To obtain this goal, it is necessary to minimize the gas pressure fo a given electron density n In a conventional positive column discharge, this can only be done at the cost of a prohibitive discharge sustaining power. The practical limit for the Q of a plasma variable reactance device using a positive column discharge has been found to be of the order of 3 at X-band, which is too low to be of practical interest. The following described device utilizing the principle of electron injection according to the invention operates at a lower gas pressure for a given electron density n and for a given discharge power than positive column and other conventional discharge devices.

'The gas discharge to be used in electron injection plasma variable reactance devices is characterized by the injection of electrons from an injection boundary in a plasma sheath into the gas to be ionized, at an energy substantially higher than the ionization potential of the gas. This electron injection takes place according to this invention through a cathode (plasma) sheath as can be seen from the potential distribution curve of FIG. 1. The electron injection energy is approximately equal to the applied discharge voltage and can therefore be externally controlled. By adjusting the electron injection energy to a value equal to or larger than about 1.5 times the ionization potential of the gas used, the following advantages are obtainable: (1) the ionization cross section is close to its maximum value; this permits operation at relatively low gas pressures, much lower than conventional discharges; and (2) most of the energy imparted to the electrons is effectively used for ionization; this results in a high ionization efficiency. Operation at low gas pressure means low electron-neutral collision frequency and high Q. High ionization efliciency means low discharge power.

An electron injection plasma variable reactance device constructed according to the invention and incorporated into a waveguide is illustrated in FIG. 2. Here, the inner surface of a waveguide structure 21 also acts as an anode to which electrons emitted by a cathode surface 23 and injected at an injection boundary of a cathode sheath adjacent the cathode surface are collected. The cathode surface 23 is insulated from the structure 21 and the potential connected between these electrodes is provided by a conventional adjustable DC voltage source, not shown. In order to prevent unwanted RF leakage from the waveguide at the position of the cathode, a waveguide choke structure 25 is utilized in a manner well known in the microwave art. An ionizable gas such as neon, xenon or krypton, to name just a few, is confined by conventional means such as quartz windows, not shown, in the ends of the waveguide in order to allow the propagation of microwave energy through the volume wherein the gas is maintained.

The potential distribution of this configuration is shown in FIG. 1 and is characterized by a cathode sheath across which practically the whole discharge voltage appears, and through which the electrons emitted by the cathode 23 are accelerated and injected into the discharge. The cathode sheath is sufiiciently thin that practically no electron-neutral collisions take place during the electron acceleration through this sheath. The gas pressure is adjusted such that the ionization mean free path for electrons at the injection energy (corresponding to the discharge voltage) is at least of the order of the distance L, which is the mean distance travelled by the electrons between the injection boundary of the cathode sheath and anode surface. Gas pressures lower than that described will result in a lower electron-neutral collision frequency and higher Q, but entails reduced ionization efficiency and higher discharge power. On the other hand, higher pressure than that described will result in a higher electronneutral collision frequency and lower Q, however, with little improvement in ionization efficiency.

In order to maintain the potential distribution of FIG. 1, the device may be operated close to temperature limited thermionic emission by, for example, adjusting the cathode temperature such that the temperature limited (saturation) cathode current does not exceed about twice the discharge current. The plasma density and reactance (dielectric constant) may be conveniently controlled by controlling the discharge voltage V in the range (above the ionization potential of the gas used) where the ionization cross section increase substantially with increasing voltage. In fact, the larger the ionization cross section for a given discharge current and voltage, the larger the rate of ion generation and the larger the resultant electron density n The choice of gas is not critical as long as the gas pressure is adjusted according to the criteria given above. For example, neon at a pressure of about 0.3 torr or xenon at a pressure of about 0.1 torr for a distance LE1 cm.

In order to reduce the discharge power required for a given plasma density n and to help localize the plasma in a well-defined column, such as column 27, a moderate axial magnetic field H can be applied. Typically, a field between 100 and 500 oersted is adequate for this purpose. By confining the plasma column, this field reduces substantially the ion loss by radial diifusion, thus reducing the power required to sustain the discharge. The confining magnetic field can be provided by a permanent magnet circuit. Because of the relatively low value of the field required, the magnetic material and the pole pieces can be incorporated in the waveguide 21, making up the walls of the waveguide in the vicinity of the plasma variable reactance device.

By using the configuration of FIG. 2, the following data has been obtained:

Waveguide Standard X-band. Gas Neon. Gas pressure 0.3 torr.

Discharge conditions for a plasma density such that the corresponding plasma frequency f is larger than the operating RF frequency of 9.4K me. were:

Discharge voltage volts 150 Discharge current milliamps Discharge power ..watts 1.5

The RF perforance was characterized by:

Quality factor Q Excess noise temperature T 100 K FIG. 3 illustrates an embodiment of the invention wherein two or more plasma variable reactance devices such as variable reactance devices 31 (similar to the variable reactance device of FIG. 2) are integrated together at appropriate locations in the same waveguide 33, in particular in the vicinity of one or several slots such as slot 35 whose radiation phase and intensity can be con trolled by electronic control of the reactance of these electron injection plasma variable reactance devices.

FIGS. 4 and 5 illustrate further embodiments of the invention. FIG. 4 indicates how the invention can be incorporated in a coaxial type waveguide structure, where a cathode 71 is a longitudinal portion of the outer wall of a center conductor 73 and the anode 75 is a corresponding longitudinal portion of the inner wall of an outer conductor 7. Of course, the cathode 71 may for the sake of simplicity be of the cold cathode type instead of the thermionic variety requiring heater leads. FIG. 5 illustrates an embodiment of the invention wherein a cathode 81 and anode 83 are disposed obliquely onopposite sides of a waveguide section 85 to thus produce an oblique column 87 having a mean distance L. The cathode 81 in this case can conveniently be of any desired type capable of the electron injection operation discussed previously. It should be understood that in all embodiments, described, proper potentials from a source not shown be connected to the cathode and anode elements. These elements may be insulated from the waveguide, as necessary, by means of conventional insulative materials such as ceramic discs 89.

With reference to FIG. 6, there is here shown an embodiment of the invention as actually constructed and operated. As can be seen, a waveguide section 101 is fitted with flanges 103 and RF windows 105 of quartz which are vacuum sealed by conventional means. Along the length of the waveguide section 101 an aperture 107 is provided about which is disposed a tubular portion 109. Within this tubular portion 109 extending from the aperture in the broad wall of the waveguide section 101 is a spiral directly heated cathode 111 connected to pins 113 extending through a glass seal 115 enclosing the extended end of the tubular portion 109. Of course, these pins 113 are adapted to be connected to a suitable source of potential to cause heating of the cathode whereby electrons are emitted in the conventional manner.

The inner portion of the waveguide section 101 was filled with neon gas at a pressure of 0.3 torr and a volt age source (not shown) of approximately volts was connected between the cathode 111 and the waveguide section 101 so as to provide anode or collector potential to the waveguide. In order to enhance the collimation of the plasma created in the device, permanent magnets and pole pieces 117 are disposed as shown, thus creating a magnetic field H bet-ween the broad walls of the waveguide.

In still another embodiment, as shown in FIG. 7, the cathode is of the indirectly heated type. This sectional view also clearly indicates the manner in which an RF choke can he incorporated about the cathode structure to prevent unwanted RF leakage. Here, a section of waveguide 151 is fitted with conventional end flanges 153 wherein are disposed RF window assemblies 155 to allow RF propagation through the waveguide 151 but prevent the leakage of gas from within the waveguide.

An aperture 157 is made in one of the broad walls of the waveguide and a tubular portion 159 is affixed therea-bout so as to house an indirectly heated cathode and choke assembly 161. The assembly 161 consists of a cathode surface 163 of a material conventionally used in vacuum tube service, a heater coil 165 adjacent the cathode surface 163 and connected to heater pins 167 extending outward of the tubular portion 159 through a gas tight insulator disc 169 of ceramic or glass, for example. The insulator disc 169 is tightly fitted into cylindrical sleeve 171, one end of which is sealed and attached to a permanent magnet pole piece 173 that is in turn attached to the extended end of the tubular portion 159. Also within the portion 159 and a part of the assembly 161 is an RF choke configuration 175 of conventional design of dimensions adapted to the particular frequency of electromagnetic energy propagating through the waveguide 151. As in the embodiment of FIG. 6, neon may be used as the gaseous medium Within the waveguide and a magnetic field may be created for collimation purposes by attaching a pole piece 177 to the opposite side of the waveguide 151 from the aperture 157 and utilizilng a permanent magnetic material for the tubular portion In practicing the invention, it is to be understood that the plasma variable reactance devices shown in the drawing as devices integrated in a waveguide can also be made as separate sealed-off tubes for insertion into independent waveguide or microwave circuit components. The only precaution to be taken in this case is to make the Wall separating the electrodes from low loss dielectric material such as a good quality glass, quartz, or low loss ceramic to avoid introducing appreciable additional RF losses due to the tube envelope.

It should also be understood that the conventional thermionic cathodes shown may be replaced by one or several conventional cold cathodes providing secondary electron emission under ion bombardment.

Furthermore, it should be noted that the magnetic field shown for the purposes of plasma confinement is optional in all embodiments and in FIG. 2, for example, may be adjusted to a value such that the electron cyclotron frequency corresponds approximately to the frequency of the RF wave or fields to be affected by the plasma varactor. Taking advantage of the cyclotron resonance permits further reductions in discharge power, gas pressure, and RF losses, and leads to a further increase in varactor Q. The magnetic field required in this configuration, however, will -be greater than that required for confinement of the plasma column only.

Still further, it should be understood that the designation of the length L as shown in the drawings is only used as an aid to indicate between which points or places the injected electrons travel and is not shown to indicate an exact path. It should further be understood in viewing the figures that the plasma sheath thicknesses 6 are much smaller than the distance L.

From the foregoing, it will be evident that the invention provides an improved and more efficient plasma variable reactance device having a relatively high Q, a very low RF insertion loss, a high average RF power handling capability, and in which the plasma is maintained substantially free from unwanted oscillations.

Although specific embodiments of the invention have been described in detail, other organizations of the embodiments shown may be made within the spirit and scope of the invention.

Accordingly, it is intended that the foregoing disclosure and drawings shall be considered only as illustrations of the principles of this invention and are not to be construed in a limiting sense.

What is claimed is:

1. An electron injection plasma variable reactance device for presenting an electronically controlled variable reactance to radio frequency electromagnetic energy, comprising:

means including an electron emitter for emitting electrons;

means including a collector surface spaced from said electron emitter for collecting the electrons emitted by said electron emitter;

a gaseous medium maintained in the region between said electron emitter and said collector surface, said gaseous medium having a pressure such that the ionization mean free path for injected electrons is at least of the order of distance L;

means connected to respective ones of said electron emitter and said collector surface for connection to an adjustable source of potential to create current flow and a plasma sheath having an injection boundary the adjustable source of potential injecting said electrons through said plasma sheath and into said gaseous medium, said electrons traveling a mean distance L from said injection boundary to said collector surface, said gaseous medium being ionized by said electrons injected by said plasma sheath to form a plasma of a density dependent upon the magnitude of the potential difference between said plasma sheath and said collector surface and upon the discharge current; and

means associated with the plasma for producing an interaction between said radio frequency electromagnetic energy and said plasma.

2. An electron injection plasma variable reactance device for presenting an electronically controlled variable reactance to radio frequency electromagnetic energy, comprising:

means including a cathode surface for emitting electrons;

means including an anode surface spaced from said cathode for collecting the electrons emitted by said cathode surface;

a gaseous medium maintained in the region between said cathode surface and said anode surface, said gaseous medium having a pressure such that the ionization mean free path for injected electrons is at least of the order of distance L;

means connected to respective ones of said cathode surface and said anode surface for connection to negative and positive terminals respectively of an adjustable source of potential to create an electron rich cathode sheath adjacent said cathode and extending to an injection boundary, said adjustable source of potential injecting said electrons through said boundary into said gaseous medium to ionize said medium to a substantially neutral plasma, said electrons traveling a mean distance L from said injection boundary through said plasma to said anode surface, said gaseous medium being ionized by said electrons injected through said cathode sheath to form a plasma of a density dependent upon the magnitude of the potential difference between said cathode sheath and said anode surface and upon the discharge current; and

means associated with the plasma for producing an interaction between said radio frequency electromagnetic energy and said plasma.

3. An electron injection plasma variable reactance device for presenting an electronically controlled variable reactance to radio frequency electromagnetic energy, comprising:

means including at least two cathode surfaces for emitting electrons;

means including anode surfaces spaced from said cathode surfaces for collecting the electrons emitted by the corresponding cathode surface;

a gaseous medium maintained in the region between said cathode and anode surfaces, said gaseous medium having a pressure such that the ionization mean free path for said injected electrons is at least of the order of the distance L from an injection boundary to said anode surfaces;

means connected to respective ones of said cathode and anode surfaces for connection to negative and positive terminals respectively of an adjustable source of potential to create a cathode sheath having a substantial potential gradient as compared to adjacent plasma adjacent each of said cathode surfaces, the cathode sheaths having injection boundaries, said adjustable source of potential injecting said electrons through said injection boundary into said gaseous plasma medium, said electrons traveling a mean distance L from said injection boundaries to said anode surfaces, said gaseous medium being ionized by said injected electrons to form a plasma of a density dependent upon the magnitude of the potential difference between said cathode sheaths and the anode surfaces and upon the discharge current; and

means associated with the plasma for producing an interaction between said radio frequency electromagnetic energy and said plasma.

4. An electron injection plasma variable reactance device for presenting an electronically controlled variable reactance to radio frequency electromagnetic energy, comprising:

means including a waveguide structure having first and second walls and a cathode surface disposed within said waveguide structure substantially flush with said first wall for emitting electrons;

means including an anode surface disposed within said waveguide structure substantially flush with said second wall, said anode surface being spaced from said cathode surface for collecting the electrons emitted by said cathode surface;

a gaseous medium maintained in the region between said cathode surface and said anode surface, said gaseous medium having a pressure such that the ionization mean free path for said injected electrons is at least of the order of the distance L;

means connected to respective ones of said cathode surface and said anode surface for connection to negative and positive terminals respectively of an adjustable source of potential to create an electron rich cathode sheath adjacent said cathode extending to an injection boundary, a substantially neutral plasma extending from said injection boundary to said anode, said cathode injecting electrons through said sheath into said gaseous plasma medium, said electrons traveling a mean distance L from said injection boundary to said anode surface, said gaseous medium being ionized by said electrons injected by said cathode sheath to form a plasma of a density dependent upon the magnitude of the potential difference between said cathode sheath and said anode surface and upon the discharge current; and

means including windows disposed at the ends of said waveguide for allowing radio frequency electromagnetic energy to propagate therethrough but restraining said gaseous medium within said waveguide.

5. An electron injection plasma variable reactance device for presenting an electronically controlled variable reactance to radio frequency electromagnetic energy, comprising:

means including a waveguide structure and at least two cathode surfaces disposed within said waveguide structure for emitting electrons;

means including anode surfaces disposed within said waveguide structure, said anode surfaces being spaced from individual ones of said cathode surface for collecting said electrons emitted by said cathode surfaces;

a gaseous medium maintained in the region between said cathode and anode surfaces, said gaseous medium having a pressure such that the ionization mean free path for injected electrons is at least of the order of the distance L where the distance L is the distance from an injection boundary to an anode;

means connected to respective ones of said cathode and anode surfaces for connection to negative and positive terminals respectively of an adjustable source of potential to create an electron rich cathode sheath adjacent each of said cathode surfaces, the cathode sheaths having injection boundaries at the juncture between the high potential gradient of the sheaths and the low potential gradient of plasma through which are injected electrons into said gaseous plasma medium, said electrons traveling a mean distance L from said injection boundaries to said anode surfaces, said gaseous plasma medium being ionized by said injected electrons to form a plasma of a density dependent upon the magnitude of the potential difference between said cathode sheaths and the anode surfaces and upon the discharge current; and

means including windows disposed at the ends of said waveguide for allowing radio frequency electromagnetic energy to propagate therethrough but restraining said gaseous medium within said waveguide.

6. An electron injection variable reactance device for presenting an electronically controlled variable reactance to radio frequency electromagnetic energy, comprising:

means including a coaxial waveguide structure having an inner conductor and an outer conductor and including a cathode surface disposed substantially flush along an outer surface of said inner conductor for emitting electrons;

means including an anode surface disposed substantially flush on an inner surface of said outer conductor for collecting electrons emitted by said cathode surface;

a gaseous medium maintained in the region between said cathode surface and said anode surface, said gaseous medium having a pressure such that the ionization mean free path for said injected electrons is at least of the order of the distance L, the distance L being from said injection boundary to said anode surface;

means connected to respective ones of said cathode surface and said anode surface for connection to negative and positive terminals respectively of an adjustable source of potential to create an electron rich cathode sheath having an injection boundary for injecting said electrons into said gaseous medium, said electrons traveling a mean distance L from said injection boundary to said anode surface, said gaseous medium being ionized by said electrons injected by said cathode sheath to form a plasma of a density dependent upon the magnitude of the potential difference between said cathode sheath and said anode surface and upon the discharge current; and

means including windows disposed at the ends of said waveguide for allowing radio frequency electromagnetic energy to propagate therethrough but restraining said gaseous medium within said waveguide.

7. An electron injection plasma variable reactance device for presenting an electronically controlled variable reactance to radio frequency electromagnetic energy, comprising:

a rectangular waveguide structure having gas-tight end sections that allow the propagation of electromagnetic energy therethrough;

a cathode surface disposed on an inner wall of said waveguide, said cathode surface being insulated from said waveguide structure and adapted to emit electrons therefrom;

an anode surface disposed on an inner wall of said waveguide spaced from said cathode surface and being adapted to collect the electrons by said cathode surface;

a gaseous medium maintained in the region between said cathode surface and said anode surface, said gaseous medium having a pressure such that the ionization mean free path for said injected electrons is at least of the order of the distance L from said injection boundary to said anode surface; and

means connected to respective ones of said cathode surface and said anode surface for connection to negative and positive terminals respectively of an adjustable source of potential to create a cathode sheath having an injection boundary at the juncture between the high potential gradient sheath and low potential "gradient plasma, electrons are injected into gaseous plasma medium, said electrons traveling a mean distance L from said injection boundary to said anode surface, said gaseous medium being ionized by said electrons injected by said cathode sheath to form a plasma of a density dependent upon the magnitude of the potential difference between said cathode sheath and said anode surface and upon the discharge current.

8. An electron injection plasma variable reactance device according to claim 7, wherein said variable reactance device also comprises magnetic field means for localizing said plasma in a well-defined column between said cathode surface and said anode surface.

9. An electron injection plasma variable reactance device for presenting an electronically controlled variable reactance to radio frequency electromagnetic energy, comprising:

a rectangular waveguide structure having gas-tight end sections that allow the propagation of electromagnetic energy therethrough;

a cathode surface disposed on an inner wall of said waveguide, said cathode surface being insulated from said waveguide structure and adapted to emit electrons therefrom;

an anode surface disposed on an inner wall of said waveguide obliquely spaced from said cathode surface and adapted to collect said electrons emitted by said cathode surface;

a gaseous medium maintained in the region between said cathode surface and said anode surface, said gaseous medium having a pressure such that the ionization mean free path for said injected electrons is at least of the order of the distance L from said injection boundary to said anode surface; and

means connected to respective ones of said cathode surface and said anode surface for connection to negative and positive terminals respectively of an adjustable source of potential to create an electron rich cathode sheath having an injection boundary for injecting said electrons into said gaseous medium, said electrons traveling a mean distance L from said injection boundary to said anode surface, said gaseous medium being ionized by said electrons injected by said cathode sheath to form a plasma of a density dependent upon the magnitude of the potential dif- 1 1 ference between said cathode sheath and said anode surface and upon the discharge current.

10. An electron injection plasma variable reactance device according to claim 9, wherein said variable reactance device also comprises magnetic field means for localizing said plasma in a well-defined column between said cathode surface and said anode surface.

11. An electron injection plasma variable reactance device according to claim 1, wherein said gaseous medium is xenon.

12. An electron injection plasma variable reactance device according to claim 1, wherein said gaseous medium is neon.

References Cited UNITED STATES PATENTS 2,813,999 11/1957 Foin 315-39 2,817,045 12/1957 Goldstein et a1. 315-39 2,837,693 6/1958 Norton 315 39 OTHER REFERENCES HERMAN KARL SAALBACH, Primary Examiner.

L. ALLAHUT, Assistant Examiner.

US. Cl. X.R. 

